Exocytosis of neurotransmitters, hormones and other cell secretory products is believed to occur by Ca ion-dependent fusion of secretory vesicles with the plasma membrane and by subsequent breakage (fission) of the barrier separating vesicle interior from the extracellular medium. We recently discovered a new protein, synexin, that mediates the calcium dependent fusion event, and we have now found that Ca ion causes the synexin molecules to assemble into cigar-shaped units which then associate side-by-side and end-to-end to form large para-crystalline arrays. We have previously shown that the chemiosmotic mechanism of chromaffin granule lysis may have useful predictive value for understanding the membrane-breakage or release process in exocytosis. We have now invented a method of exactly quantitating proton entry into granules by 31P nuclear magnetic resonance spectroscopy, and in so doing have discovered that the ATP inside chromaffin granules (concentration approximately 0.15M) was in a conformation different from that of ATP in simple solution. In exocytosis from chromaffin cells we propose that intracellular ATP induces entry of H+ into fused granules, and that Cl- from outside the cell also enters the granule and thus causes the local fused granules to undergo outward breakage (fission) and release.